Color correction is a term most often applied to a technique or techniques for modifying color pictures for printing. With the advent of color electronic printing technologies, this technique has become increasingly important. There are two systems commonly used for production of color images: the additive color system and the subtractive color system. In the additive color system, colors are produced by the generation of light. To obtain a particular color in the additive color system, one adds more or less of one of the additive primary colors: red, green, and blue. Additive color systems are used most often in self-luminous color systems, such as cathode-ray-tubes and projection television.
On the other hand, subtractive color systems produce colors by selecting absorption of portions of the spectrum of an incident light source by certain dyes and pigments. Printing systems are primarily subtractive color systems. Cyan, magenta and yellow are the primary subtractive colors.
Ideally, when incident light is reflected off the pigment cyan, the red is subtracted. In other words, the pigment cyan subtracts all the red color out of light. In the same manner, the pigment magenta is related to green and the pigment yellow is related to blue.
In color electronic printing, differing quantities of cyan, magenta and yellow pigments are deposited on the final image receptor to create a color. Therefore, the resultant color is dependent upon the relative densities of the deposited pigments. Unfortunately, because color pigments are not ideal, the resultant color created rarely matches it's original. For example, when magenta is used, more than green is subtracted from white. Typically, magenta printing dyes subtract some amount of blue light and therefore can be considered to contain some yellow contamination. Because of these imperfections in the spectral content of toner pigments, the presence of "color contaminants" can cause an uncorrected reproduction to have hue shifts and often appear "muddy" or dark.
The basic problems associated with imperfect dyes have been recognized by others. In 1937, Neugebauer developed mathematical expressions to compensate for the color contaminants found in printing dyes. These expressions were based on the assumption that the image is comprised of colored dots or pixels of varying area which are randomly distributed over the image. Using the Neugebauer equations, the fractional areas of each colorant required to produce the original color were found.
The emergence of matrix imaging technologies has given new interest in developing halftoning techniques to suit these systems. Various types of matrix color printing technologies include thermal transfer, ink jet, and rasterscan electrophotography. In these systems, an image is again divided into very small dots, called pixels. The greater the number of pixels per inch, the higher the resolution of the resultant reproduction. Matrix technologies are unique in their ability to place dots of various colors, with identical area, in the same location. At each pixel, the color to be printed is composed of 1 to 3 dots from the substractive pigments placed on top of each other. However, because of this differing image structure, that is, constant instead of varying pixel area, the assumptions on which the Neugebauer equations were based are no longer valid.
Correct color reproduction is achieved if the original and reproduced colors match in lightness, (dark to light quality of the color) hue (blue, green, red, etc.), and saturation, (amount of gray present in a color). If the visible (non-filtered) density of two separate colors is identical, then their lightness is said to be equal. This is easily seen by a linear 1:1 tone reproduction curve ("TRC"). A TRC is the ploy of output density of the reproduction versus input density of the original. If the densities of two colors are similar as a function of wavelength across the visible spectrum, those colors are said to match hue and saturation as well as lightness.
Therefore, the cental nature of problems with maintenance of color integrity can arise from spectral impurities in the toner pigments and non-linear TRCs which are a result of both toner pigment characteristics and development/transfer characteristics. The term "development/transfer characteristics" refers to the entire system transfer function of characteristics between the input signal to a printing device and the final printed output. For example, the input signal to a laser source used for selectively discharging pixel areas of the photoreceptor of the machine and the efficiency of transfer of toner particles to the photoreceptor, and subsequent transfer of the toner particles to the image receptor to provide a final physical density of toner on the ultimate image receptor.
It should also be noted that the fusing characteristics of the plastic materials used in toners can effect the toner reproduction curve. The length of time the toner materials are in the fuser and the fuser temperature (that is, the amount of heat used to fuse the materials) can affect the TRC for a given set of toner materials. Thus, it should be noted when different types of image receptors are used which require different fuser heat transfer characteristics, the color characteristics of the resultant image can change. For example, if one has to use a subtractive color in an electrophotographic printing apparatus to create color transparencies for use on an overhead projector, most of the materials used for such transparencies require different fuser temperatures than most papers used in such machines.
In conventional printing processes, the visible spectrum is divided into three broad regions, namely, blue (400-500 nm), green (500-600 nm) and red (600-700 nm) and three density values from each region are measured (called the RGB value of the color). These filters correspond to the colorant pigments, cyan, magenta and yellow, respectively. According to Yule, since these separation filters are broadband (approximately 100 nm each), the additivity rule of densities applies. Simply stated, the additivity rule states that when several colorants are superimposed, the density of the combination is equal to the sum of the densities of the individual colorants. In mathematical form, this rule could be stated as follows: EQU Ro=f1(C)+f2(M)+f3(Y) EQU Go=f4(C)+f5(M)+f6(Y) EQU Bo=f7(C)+f8(M)+f9(Y)
where
Bo, Go, Bo=Original color PA0 C=Cyan PA0 M=Magenta PA0 Y=Yellow PA0 f1 . . . f3=(n) density through the Red filter where f2 and f3 are non-complement densities PA0 f4. . . f6=(n) density through the Green filter where f4 and f6 are non-complement densities PA0 f7 . . . f9=(n) density through the Blue filter where f7 and f8 are non-complement densities
The TRCs generated by real implementations of printers using real toner pigments have signficant regions of linearity, but suffer from significant non-linearities, primarily at points on the TRC curve of low density and where the density approaches its saturation level. These regions of non-linearity require correction to provide correct lightness rendition between original and reproduction. Correct hue and saturation rendition suffer from the "color contaminants" inherent in conventional printing pigments. For ideal toner pigments, the red output density would only be a function of the cyan pigment, the green output density would only be a function of magenta pigment and the blue output density would only be a function of the yellow pigment. Thus, those terms in the equations listed above with non-complement densities would tend to zero and drop out. Therefore, non-zero values for the non-complement densities represent the spectral impurities in the colorants. Furthermore, for a given subtractive color printing machine using a give set of toners, the f terms in each of the equations vary as a function of toner density and other factors which affect the resultant image color, such as heat transfer in the fuser. Naturally, the f values vary as a function of the pigment characteristics of the toner set being used. Non-linear TRC curves for the three colorants lead to different values for the f terms. Hence, measurement of the colorant densities along the TRC curve to create a number of f terms is required.
The system proposed by the Additivity Rule requires significant mathematical computation which creates real time design problems or slow image processing rates. In essence, the inventors of the present invention have discovered that the system appears to provide good first approximations of color correction signals. The insight of the present inventors was essentially to note what needs comparing, at any of a given set of densities, is how closely the actual resultant image RGB color elements approximate the desired R.sub.i, G.sub.i, B.sub.i input signals at each point in RGB color space. As used here in the concept of RGB color space is a mathematical three-dimensional space of varying values of red image spectral content, or red color value, green color value, and blue color value of the image. Red, green and blue filters of high spectral purity are much easier to create than complimentary color toner pigments of similar purity. Red, green and blue input signals to a laser printer can be created to virtually any desired precision. Therefore, it is the comparison of the input color values to the resultant output color values of the image which needs to be addressed for high quality color correction, particularly in the non-linear regions of the tone reproduction curves for the various pigment transfer and developing apparatus for a given subtractive color printing machine.